Sorbus alnifolia

Photos by Mark Dwyer

It’s rare to be able to make the claim that you have actively observed the same tree up close almost daily for many years. I can only make this claim for the Korean mountain ash that I’ve enjoyed observing for the last 12 years along our dog walking route in Janesville, Wisconsin. I’ve seen the transition from dormancy to amazing white flower clusters in spring, enjoyed the glossy green foliage that ultimately turns shades of yellow and orange in fall and noted the progression of the colorful and persistent fruiting from up close. I feel that I know this plant and recommend it is an excellent candidate for a smaller tree specimen that really offers ornamental contributions throughout the year.

Native to Asia (Japan, Korea, China and Taiwan), this member of the Rosaceae family is densely branched and while pyramidal in youth, does develop a consistently symmetrical, oval form as it matures. The variety ‘Red Bird’ is a very narrow selection with all the same attributes. While tolerant of part shade, the best flowering and fruiting displays are observed in full sun situations. This species in not urban or pollution tolerant so minimize stress by providing adequate open space for root development which will maximize health and longevity. Korean mountain ash is mildly drought tolerant once established although well-drained soils are required. This species grows well in slightly acidic to slightly alkaline soils (pH 5-8). The growth rate of this species would be described as moderate to fast.

The smooth, grey, beech-like bark is finely striped and notable, particularly during the winter months. The creamy white, flat-topped flower clusters in late spring attract bees and butterflies and the fruits form quickly but mature well into autumn and hang in loose, colorful groupings. These fruits (tiny, globose pomes) are typically an orange-red to scarlet color (rarely yellow) and are very persistent on the tree. Migrating and local birds certainly enjoy the feast. The simple, beech-like leaves transition to an impressive yellow to golden-orange in autumn. Fall color will vary but is consistently impressive. Pruning is rarely needed although it can be accomplished in late fall through early spring. The dense branching of this species does create heavy shade under the canopy.

Korean mountain ash is significantly less susceptible to the range of diseases that afflict other Sorbus species. Furthermore, this species is very resistant to the borers and other pests that visit other mountain ash species. However, stressed specimens will be more prone to displaying symptoms of bacterial fireblight, scab, cankers, crown gall, powdery mildew and rust. Propagation is most common by seed but can also be accomplished with softwood cuttings or grafting.

Why Grow Sorbus alnifolia?

• Showy white flowers in late spring
• Amazing fall color peaking at orange
• Nice smaller-scaled, durable specimen
• Showy and persistent fruits (birds!)
• More disease/pest resistant than other Sorbus sp.